The present invention is directed to a method for regulating a circumferential register in a web-fed rotary printing press. An actual web tension passing through at least two print units of the press is used to regulate the circumferential register.
A regulation of the transverse and linear register in a press is known from EP 0 951 993 A1. The correction of the transverse register is performed by use of a traction roller. A change in the web""s linear expansion is determined by use of web speeds and other parameters, such as tension-expansion diagrams and set values of tension. This change is transmitted back to regulation arrangements for cylinders of the printing groups and/or for an actuator for setting and correcting the cutting register.
WO 00/34042 discloses a method for regulating a register. A deviation of a web tension from a set value changes the number of revolutions and/or the angular position of a first printing location in relation to a second printing location.
EP 0 950 519 A1 discloses a method for regulating the cutting and inking registers of several webs in respect to each other, as well as between individual print units. Expansions of the webs are determined by use of the operating points of the drive mechanisms of conveying rollers, active cylinders and traction rollers. Correcting values determined from this are added as position offsets to the command variables for the cutting register or the inking register.
The object of the present invention is directed to providing methods for regulating a circumferential register in a web-fed rotary printing press.
In accordance with the present invention, this object is attained by providing at least two printing units in the web-fed rotary press through which the web passes sequentially. An actual value of the tension in the web is determined. An angular position or speed of one cylinder of a first print unit, with respect to a cylinder of a second print unit, is regulated. The web tension is determined downstream of the last print unit, in the production direction. This tension is used to regulate the angular position or speed of a cylinder in a print unit upstream of this last print unit. A difference between the actual web tension and a set value can be determined and used to regulate the upstream cylinder. The change value of the cylinder may be formed as a function of web quality.
The advantages which can be obtained by the present invention reside, in particular, in that an error in the circumferential register, which error is the result of web tension fluctuations or of fluctuations of the circumference of the printing cylinder caused, for example, by brief changes in the thickness of the web, is counteracted without a great outlay for measuring and regulating. The error to be corrected can be a register offset, which is an error in the so-called inking register, or a register error between the two sides of the imprinted web. A difference in the conveying speed of the web occurring between two print positions is corrected by a change of the relative rotary position of the two print positions with respect to each other.
It is a great advantage of the present invention that it is possible to omit elaborate measurements and regulations by the use of ink markers applied to the web. Neither a direct measurement of the web expansion, nor a regulation of the web tension or torque by use of the operating points of the drive mechanisms of the print units, for the purpose of a correction, needs to be performed.
The method, and a corresponding device in accordance with the present invention, can be employed particularly advantageously in case of a register offset in print units with several double printing groups arranged one behind the other, or with one on top of the other for a rubber-against-rubber operation, such as in an eight-unit printing tower composed of two so-called H-print units, wherein at least two of the printing locations make successive imprints on the same location of a web.
For eight-unit towers, consisting of two H-printing groups, it is particularly advantageous to evaluate the chronological change of the tension of the web, for example before or after passing through a gluing station for the web, downstream of the last printing group, or the last print unit, and to perform only a change of the angular position of the H-print unit which is first in the running direction of the web in relation to the second H-print unit. This regulation of the circumferential register, and in particular of the ink register, is advantageous for several, for example for at least three print units through which a web successively passes, wherein a distance between the two first printing nips is clearly less than a subsequent distance from a third printing nip. An eight-unit tower, consisting of two H-print units, is an example for this.
The method in accordance with the present invention can be advantageously used for regulating the register between the two sides of the web in printing presses having satellite print units.
In one embodiment, the present invention is of particular advantage in that a chronological change of the tension downstream of the last print unit, i.e. a difference in the tension at a first point in time and at a second point in time, is used directly and linearly for the correction of the register offset of the second H-print unit in relation to the first H-print unit. This requires an extremely low measuring and regulating outlay, while still being sufficiently accurate.
It is possible, in an advantageous manner, to omit a tension regulation, for example by use of a moment-controlled traction roller downstream of the printing tower. An unregulated traction roller, with a controlled number of revolutions, is sufficient here. A register offset, for example caused in a gluing station, is corrected in an advantageous manner exclusively by use of the circumferential register of the cylinders, in particular the cylinders of the first print unit, or of the first two print units. It is possible, in particular, to regulate the two first print units, for example in the form of an H-print unit, together, so that it is merely necessary to determine a single correction value and to perform a single correction.